OF ARTS AND SCIENCES. 127 



than is obtained from a junction of bars of antimony and bismuth 

 under similar circumstances. Finally, to employ a condensing mirror 

 having the junction at its focus is simpler than to multiply the number 

 of junctions, and, moreover, it adds nothing to the resistance of the 

 circuit. 



Instruments. 



The Thermograph. 



There will now be described, in detail, the instrument which was 

 the outgrowth of the principles touched upon above. It is not beyond 

 the power of one possessed of very ordinary mechanical skill, and who 

 may be perforce his own instrument-maker in matters like the present. 

 The measures will be given in inches, since most workmen still employ 

 that unit. 



A cvlinder is turned from vulcanite, or some hard wood. 3 in. lon<» 

 and 1 in. in diameter. For f in. at one end this cylinder is made of 

 less diameter by -^ s in., in order that it may receive and hold firmly a 

 piece of brass tubing 3 in. long, 1 in. in external diameter, and ^. y in. 

 thick. The end of the cylinder carrying the tube will be designated 

 in what follows as the " front end." The cylinder is next pierced, in 

 the direction of its length, with two J in. holes lying upon the same 

 diameter and ^- e in. apart. Through these holes copper rods are firmly 

 driven, being left to project -J- in. above the front end of the cylinder. 

 They are allowed to project f in. from the back end of the cylinder, 

 and are afterwards bent apart somewhat to facilitate the attachment of 

 screw cups. The front ends of the rods are made somewhat smaller 

 than the other portion by filing. 



The thermal junction is made as follows. Some bits of rather 

 wide watch-spring are procured, together with some rolled sheet nickel. 

 A strip of the same width as that of the watch-spring is cut from the 

 nickel, and, the ends of the spring and of the piece of nickel having 

 been scarfed with a file, the two are united by means of the strongest 

 hard solder obtainable. For convenience in handling, it is well to 

 leave the piece of watch-spring long. The compound strip can now 

 be made straight upon the edges, and worked thin with a line file. 

 Trial is occasionally made with a screw caliper, and when the thick- 

 ness has been reduced to .02 mm. it is best to discontinue. The 

 solder junction must be very perfect, or it will be pulled apart in the 

 filing. With great care and patience the thickness of the strip of 

 metal can be reduced to .01 mm., but there is no compensating ad- 

 vantage for the labor expended. When finished, the wide strip can 



